In the case of bipolar transistors, the extrinsic base resistance, with the transition frequency and base collector capacitance, is one of the decisive transistor parameters which determine important characteristic quantities such as the maximum oscillation frequency, the amplification (“gain”), the minimum noise factor, gate delay times, etc. of the bipolar transistor. The extrinsic base resistance corresponds to the resistance between the base, or the actual base area, and an external contact, which is connected to the base via a connecting line.
Regarding the above-mentioned transistor parameters, the following applies, for instance, to the maximum oscillation frequency ƒmax of the bipolar transistor:
                              f          max                ≈                                            f              T                                      8              ⁢                              π                ·                                  R                  B                                ·                                  C                  BC                                                                                        (        1        )            where ƒT is the transition frequency, RB is the extrinsic base resistance and CBC is the base collector capacitance of the bipolar transistor.
For the minimum noise factor Fmin of a bipolar transistor, the following applies, depending on the extrinsic base resistance RB and frequency ƒ:
                              F          min                ≈                  1          +                      1            β                    +                                    f                              F                T                                      ·                                                                                                      2                      ·                                              I                        C                                                                                    V                      T                                                        ·                                      R                    B                                    ·                                      (                                          1                      +                                                                        f                          T                          2                                                                          β                          ·                                                      f                            2                                                                                                                )                                                  +                                                      f                    T                    2                                                        β                    ·                                          f                      2                                                                                                                              (        2        )            where β is the small signal current amplification, IC is the collector current, and VT is the thermal voltage of the bipolar transistor.
From the two formulae (1) and (2), it can be seen that the extrinsic base resistance RB should be small for fast switching and low noise factors. One method of reducing losses in the case of bipolar transistors is the use of a polysilicon electrode for contacting the base. A p+ polysilicon layer provides a low-resistance path with correspondingly low capacitance for the base current.
Specially small extrinsic base resistances can be achieved, for instance, by use of the concept of the so-called “self-aligned double polysilicon bipolar transistor”, as described in “Self-Aligned Bipolar Transistors for High-Performance and Low-Power-Delay VLSI”, T. H. Ning et al., IEEE Transactions on Electron Devices, Vol. ED-28, No. 9, pp. 1010-1013, 1981. This concept is therefore used in almost all widely used production technologies for ultra-high frequency bipolar transistors.